This is a real simple and quick build log that I did this morning.
I was sweeping up the dogs fur when I remembered I had a roomba a few years back and stopped using because the stock batteries sucked. I had been toying around with the idea of making it lipo powered but never got around to it. Its so simple that I'm kicking myself for not doing it sooner.
So the stock battery is 14.4v Nicd so a 4s lipo makes a decent replacement. I tried taking it apart the stock battery with my triangle security bits, but it wasn't happening. Only one screw came out and the rest just started stripping.
My 4S lipo is made of two 2S 5000mah 20C lipos wired in series. These are about $13 a piece from Hobbyking.com or other international online stores.
As you can see on the stock battery, there are 3 connections. Two are negative and one is positive. What their exact purposes are, I don't know, and don't need to know. I just made sure I soldered the connections up the same on the Roomba lipo battery wires. The wires then passes through a hole I drilled through the stock battery location and exits under the flap on the top. I made the wires a tad short, but it works just fine.
The batteries are mounted on top and in the front. I had them towards the back, but then Roomba was popping a wheelie the whole time. The stock battery is very heavy and in the front, so the lipos also need to be in the front. Sticky back velcro is what holds them in place.
Lipos should not be discharged to low, so while testing mine I have a lipo balancer tester plugged into one of my batteries balance plugs. I've been keeping an eye on it and don't want the batteries to drop below 3.4v per cell. The Roomba has ran for well over an hour and the batteries are resting at 3.65v per cell.
Charging these lipos cannot be done with the stock charger. You have to use one purposed for lipo charging. These can be had for $20 to $200 depending on how advanced you want to be. Charging both batteries separately would take 2 hours total.
Again, if you use the stock charger with the lipo batteries, they will go BOOM and you will have a fire to deal with.
March 24th update:
I freshly charged the packs, placed them on my Roomba and set it to MAX which means run until the batteries are drained. I also attatched a Lipo Alarm which beeps once the lipos reach ~3.4v per cell. I did this test for two reasons. One, I wanted to see how long it would go. Two, I wanted to see if the Roomba's built in cutoff would happen before the lipo alarm went off.
Results: The Roomba ran for over two and a half hours. The Roomba's built in light was red, but it did not stop running before the Lipo Alarm went off. This means you will need an external lipo monitor to safely use lipos on your Roomba. The one I used cost $3 from Hobbyking.com